1. Field of the Invention
The present invention relates generally to the field of heat transfer. More particularly, the present invention relates to wicks for use in loop heat pipe evaporators.
2. State of the Art
There are numerous instances where it is desirable to transfer heat from a region of excess heat generation to a region where there is too little heat. The object is to keep the region of heat generation from getting too hot, or to keep the cooler region from getting too cold. This is a typical thermal engineering problem encountered in a wide range of applications including building environmental conditioning systems, spacecraft thermal control systems, the human body, and electronics.
A variety of techniques can be employed to achieve this heat sharing effect. These include heat straps (simple strips of high conductivity material), closed loops of pumped single-phase fluid, heat pipes, mechanically pumped two-phase loops, and capillary pumped two-phase loops.
The most advanced and efficient concept is the capillary pumped two-phase loop and the related loop heat pipe (LHP). LHP technology has recently been developed for spacecraft applications due to its very low weight to heat transferred ratio, high reliability, and inherent simplicity.
A LHP is a two-phase heat transfer system. The LHP is a continuous loop in which both the vapor and the liquid always flow in the same direction. Heat is absorbed by evaporation of a liquid-phase working fluid at the evaporator section, transported via the vaporized fluid in tubing to a condenser section to be removed by condensation at the condenser. This process makes use of a fluid's latent heat of vaporization/condensation, which permits the transfer of relatively large quantities of heat with small amounts of fluid and negligible temperature drops. A variety of fluids including ammonia, water, freon, liquid metals, and cryogenic fluids have been found to be suitable for LHP systems. The basic LHP consists of an evaporator section with a capillary wick structure, of a pair of tubes (one of the tubes is for supply of fluid in its liquid state, and the other is for vapor transport), and a condenser section. In many applications, the pressure head generated by the capillary wick structure provides sufficient force to circulate the working fluid throughout the loop, even against gravity. In other applications, however, the pressure differential due to fluid frictional losses, static height differentials, or other forces may be too great to allow for proper heat transfer. In these situations it is desirable to include a mechanical pump to assist in fluid movement. Systems employing such pumps are called hybrid capillary pumped loops.
In designing LHP evaporators, the art has long taught the use of cylindrical geometry, particularly for use in containing high-pressure working fluids, such as ammonia. Referring to FIGS. 1-3, prior art evaporators 10, 30, and 50 are illustrated as having a cylindrical geometry, where a wick 4 has a central flow channel 2 and is surrounded at its periphery by a plurality of peripheral flow channels or vapor grooves 6. Capillary evaporators having a central flow channel 2 in the wick 4 are sensitive to a problem called back-conduction.
Back-conduction in capillary evaporators refers to the heat transfer due to a temperature gradient across the wick structure, between the vapor grooves 6 in the evaporator and the liquid that is returning to the evaporator in the central flow channel 2.
This energy is normally balanced by sub-cooled liquid return and/or heat exchange at the hydro-accumulator in the case of loop heat pipes. Refer to J. Ku, “Operational Characteristics of Loop Heat Pipes,” SAE paper 99-01-2007, 29th International Conference on Environmental Systems, Denver, Colo., Jul. 12-15, 1999, which is incorporated herein by reference in its entirety.
It would be beneficial to minimize back-conduction for several reasons. First, decreased back-conduction would permit minimization, or even elimination, of liquid return sub-cooling requirements. Second, decreased back-conduction would allow the evaporator operating temperature to approach heat sink temperature, particularly at low power. Third, decreased back-conduction would allow loop heat pipes to operate at low vapor pressure, where the low slope of the vapor pressure curve allows small pressure differences in the loop to result in large temperature gradients across the wick. Finally, decreased back-conduction would minimize sensitivity to adverse elevation.
Thus, what is needed is a wick for use in a LHP evaporator that has improved back-conduction performance.
Aside from any back-conduction considerations, another inherent disadvantage of the cylindrical evaporator is its cylindrical geometry, since many cooling applications call for transferring heat away from a heat source having a flat surface. This presents a challenge of how to provide for good heat transfer between the curved housing of a cylindrical evaporator and a flat-surfaced heat source.
Typically, the evaporator housing is integrated with a flat saddle to match the footprint of the heat source and the surface temperature of the saddle is dependent upon the fin efficiency of the design. FIG. 1 shows a prior art cylindrical evaporator 10 (cross-sectional perspective view) integrated with a single saddle 20 for mounting to a single, flat-surface heat source (not shown). Heat energy is received via a single heat input surface 22. FIG. 3 shows an alternative design for a prior art cylindrical evaporator 30 (cross-sectional perspective view) integrated with a single saddle 40 that has extended fins. Heat energy is received via a single heat input surface 42. FIG. 2 shows a prior art cylindrical evaporator 50 (cross-sectional perspective view) integrated with two saddles 60, 70. Heat energy is received via two opposed heat input surfaces 62, 72.
For large heat sources, requiring isothermal surfaces, multiple evaporators are often required. The number of required evaporators would also increase as the thickness of the envelope available for integrating the evaporator (i.e., the distance between the heat input surface 22 and the bottom 24 of the evaporator 10 of FIG. 1, or the distance between the opposed heat input surfaces 62, 72 of the evaporator 50 of FIG. 2) decreases. That is because the width of the cylindrical evaporator is a function of the evaporator diameter and the diameter is limited to integration thickness. Increasing the number of evaporators increases the cost and complexity of the heat transport system.
Capillary evaporators with flat geometry have been devised, which match a heat source having rectangular geometry. Flat geometry eliminates the need for a saddle and avoids the inherent thickness restraints currently imposed upon cylindrical capillary evaporators.
The art of flat capillary evaporators for use with high-pressure working fluids teaches use of structural supports for resisting any deformation forces exerted thereon due to the pressure of the working fluid. The plates are sealed together, which often requires use of bulky clamps or thick plates. Clamps, thick plates and added support mechanisms have the disadvantages of unnecessary weight, thickness and complexity.
U.S. Pat. No. 5,002,122 issued to Sarraf et al., and titled “Tunnel Artery Wick for High Power Density Surfaces,” relates to the construction of an evaporator region of a heat pipe, having a flat surface 12 for absorbing high power densities. Control of thermally induced strain on the heated surface 12 is accomplished by an array of supports 14 protruding through the sintered wick layer 18 from the back side of the heated surface and abutting against a heavier supporting structure 16. The sintered wicks 18 are taught as being made from silicon and glass. The supports 14 protruding through the wick 18 are bonded to the plate 12 to provide the necessary support.
U.S. Pat. No. 4,503,483 issued to Basiulis, and titled “Heat Pipe Cooling Module for High Power Circuit Boards,” is directed to a heat pipe having an evaporator section configured as a flat pipe module 22 for attaching directly to electronic components 28. This evaporator assembly sandwiches two wicks 36 between two opposing plates 34. (Refer to FIG. 4.) Basiulis teaches use of a central separator plate 38 having bars 40, which solidly connect the opposing plates 34 to provide strength and prevent mechanical deformation. Refer to col. 3, lines 3-11.
U.S. Pat. No. 4,770,238 issued to Owen, and titled “Capillary Heat Transport and Fluid Management Device,” is directed to a heat transport device with a main liquid channel 22 and vapor channels 24, 26, 32, 34 containing wick material 36. The liquid channel 22 and vapor channels 24, 26, 32, 34 are disposed between flat, heat conducting plate surfaces 14, 16. The plates 14, 16 are separated by ribs 38, 40, 42, 44 having a thickness that provides structural stiffness.
U.S. Pat. No. 4,046,190 issued to Marcus et al., and titled “Flat Plate Heat Pipe,” relates to flat plate vapor chamber heat pipes having two flat plates 2, 3 sealed together in parallel planes. Spacing studs 4 are aligned at regular intervals to provide structural support for the plates 2, 3, as well as to serve as an anchor for metal wicking 5.
U.S. Pat. No. 4,685,512 issued to Edelstein et al., and titled “Capillary-Pumped Heat Transfer Panel and System,” discloses a capillary-pumped heat transfer panel having two plates and a wick. Each plate has a network of grooves for fluid communication with a liquid line, and thus has corresponding non-groove portions that form the thick walls of the grooves on the interior surface of the plate. When the plates are sealed together, these non-groove portions, which form the walls of the grooves and have very substantial thickness relative to the wick material, serve the function of supporting structures for the assembly.
The main disadvantages of support structures such as studs, bars, ribs, and the like, (i.e., Sarraf et al., Basiulis, Marcus et al., and Owen) and bulky walls (i.e., Edelstein et al.) are that they add weight to the evaporators. Flat plate evaporators without support structures are known in the prior art, but are useful only in relatively low pressure systems so as to avoid deformation of the unsupported flat plates, which would be the natural result of pressure forces exerted by high-pressure working fluids, such as ammonia.
U.S. Pat. No. 3,490,718 issued to Vary, and titled “Capillary Radiator,” teaches capillary type radiator construction that is flexible or foldable. This patent discloses an embodiment without use of an intermediate spacer means for forming the capillary passages, and thus no separate support is provided for the plates of this embodiment. Vary teaches, however, that a radiator mechanism based on this concept must be in a relatively low pressure system in which the combined header and vapor pressures remain below about 10 psia.
U.S. Pat. No. 5,642,776 issued to Meyer, IV et al., and titled “Electrically Insulated Envelope Heat Pipe,” is essentially a heat pipe in the form of a simple foil envelope. Two plastic coated metal foil sheets are sealed together on all four edges to enclose a wick that is a semi-rigid sheet of plastic foam with channels cut in its surfaces. The disclosed working fluid is water, a relatively low-pressure working fluid. The Meyer, IV et al. disclosure does not address the issues of containment of high-pressure working fluids in flat capillary evaporators.
Thus, there is a need for a flat capillary evaporator that has the structural integrity to accommodate high-pressure working fluids, while avoiding the bulky mass of support structures such as ribs or thick walls.
In many terrestrial applications, including electronics, heat is dissipated from a heat source via a passive heat sink, a heat sink aided by a fan, or other conventional means. The conventional schemes do not have the low weight to heat transferred ratio characteristic of LHP technology. Unfortunately, prior art LHPs have not provided for a way to reduce back-conduction, which is often largely due to the hydrostatic pressure caused by height differentials that arise in terrestrial applications. The temperature gradient across the wick is directly proportional to the pressure difference across the wick. That is to say, gravity causes hydrostatic pressure, which increases the temperature gradient across the wick, which increases back-conduction, and high back-conduction limits LHP design choices by requiring high-pressure working fluids. This excludes water (a desirable choice) and other low-pressure fluids as a practical choice for terrestrial applications.
Thus, what is needed is a LHP that can operate under terrestrial conditions with reduced back-conduction.
Prior art LHPs are bulky, with an evaporator and condenser that tend to be physically distanced from one another. However, these prior art LHP configurations are not well suited for applications where the heat input surface and the heat output surface are intimately close to one another.
Thus, what is needed is a LHP that is physically compact with the various components integrated into a unitary package.